Secondary batteries are batteries in which the chemical reaction that generates electrical energy is electrically reversible. Commonly used secondary cell (“rechargeable battery”) chemistries are lead-acid (such as a conventional automobile battery), nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium ion (Li-ion), and lithium ion polymer (Li-ion polymer). These batteries offer the benefit of repeated use and recharging, thereby extending the life of the battery as compared to a conventional primary battery, in which the electricity-producing chemical reaction is a one-way reaction that eventually consumes the component materials.
In recent years, the use of lithium-ion (Li-ion) batteries has expanded from small appliance applications to larger scale applications. Most recently, Li-ion batteries have been considered for use in electric vehicles, such as gas-electric hybrid automobiles and electric-only automobiles. Although such batteries are effective in many of these applications, insufficient thought appears to have been given to the availability of lithium, as compared to the world demand for lithium for use in batteries for conventional purposes, as well as for transportation.
World reserves of lithium are currently estimated to be on the order of about twenty-eight million tons, making lithium one of the more uncommon elements on the Earth's surface. Only about four hundred thousand tons of lithium are located within the borders of the United States, with the bulk of the world's reserves are located within Bolivia, China, and Russia. As demand for lithium continues to increase worldwide, one can readily predict that other countries and regions may seek to control their supply of lithium, either for their own internal use or to inflate market price.
To have adequate capacity for practical use by consumers, a typical plug-in hybrid car requires a battery pack weighing at least six hundred pounds, at least forty pounds of which is lithium. To convert the entire population of three hundred million automobiles in the United States to a lithium-based battery system would require about six million tons of lithium, exceeding the U.S. supply of lithium by a factor of fifteen to one.
Extrapolating the situation worldwide to a population of roughly eighteen times that of the United States, a conservative estimate for lithium demand would be approximately sixty million tons, thus exceeding the global supply of lithium by a factor of more than two to one.
Because lithium is fairly rare, some battery manufacturers have sought to produce effective battery systems using more abundant materials, such as magnesium, which has been explored with some degree of success. Magnesium batteries have substantial promise as rechargeable systems for many battery applications, including the electric car, portable electronics, and tools. Whereas alkali metals (such as lithium) are highly flammable and may be poisonous, alkaline earth metals (such as magnesium) are easy to process and exhibit stable behavior. Additionally, magnesium is the third most common metal that can remain unprotected in the Earth's atmosphere, with the world reserves of magnesium being on the order of at least eight billion tons. Moreover, vast amounts of magnesium salts are dissolved in sea water.
U.S. Pat. No. 6,316,141 describes a magnesium battery that has a magnesium anode, a molybdenum-containing intercalation cathode, and a non-aqueous electrolyte comprising an organic solvent and an electrolytically active salt. Potential shortcomings of this battery, however, are the limited worldwide availability of molybdenum, the use of potentially hazardous solvents, and design constraints associated with a liquid electrolyte.
As compared to a lithium battery, a magnesium battery may require approximately twice as much metal—that is, about eighty pounds per battery pack. With this greater requirement, the world demand for magnesium batteries (for vehicle usage alone) could possibly reach as much as twelve million tons. However, because of the abundance of magnesium, the global supply of magnesium far exceeds the demand by a factor of at least six hundred fifty to one.
Another metal that has been investigated for use in batteries as an alternative to lithium is sodium. Sodium is attractive for such use because of its high reduction potential, its low weight, its non-toxic nature, its relative abundance and ready availability, and its low cost. In order to construct practical batteries, the sodium must be used in liquid, or molten, form and must be kept isolated from moisture, including humidity in ambient air.
An example of a commercially viable sodium battery was developed in 1985 by the Council for Scientific and Industrial Research in Pretoria, South Africa. The battery, which was invented by the Council's Zeolite Battery Research Africa Project (nicknamed “ZEBRA”), is described, among other places, in U.S. Pat. No. 4,975,344, the entire disclosure of which is hereby incorporated by reference. The ZEBRA battery includes an anode of molten sodium, an electrolyte of molten sodium chloroaluminate (NaAlCl4), a cathode of nickel or another transition metal in the discharged state (a metal chloride when charged), and a ceramic separator of sodium ion-conducting beta-alumina to prevent contact between the molten sodium anode and the NaAlCl4 electrolyte. The technical name for the battery is sodium-nickel chloride (Na—NiCl2) battery, but it is commonly referred to as the “ZEBRA battery.”
The ZEBRA battery's liquid electrolyte solidifies below its melting point of 157° C. (314.6° F.), and the normal operating temperature range is typically between 250° C. (482° F.) and 350° C. (662° F.). The β-alumina solid electrolyte (BASE) that is employed as a membrane, or separator, within this system is very stable, both to the sodium metal anode and the sodium chloroaluminate electrolyte. The primary elements used in the manufacture of ZEBRA batteries—that is, sodium, chloride, and aluminum—have much higher worldwide reserves and annual production than the lithium used in lithium-ion batteries discussed above.
One potential shortcoming of the ZEBRA battery, which may prevent its widespread adoption, is the reliance upon nickel as the cathode material. The estimated world reserves of nickel are on the order of about eight hundred million tons, closer to those of lithium (twenty-eight million tons) than magnesium (eight billion tons). Worldwide adoption of the ZEBRA battery as the primary type of automotive battery would quickly deplete the available global reserves, thereby reinforcing the need for a battery that utilizes more abundant materials.
Accordingly, a need exists in the industry for a durable battery made of readily abundant materials, which may be easily assembled and repeatedly recharged. The present disclosure addresses such need.